Advances in breast cancer screening and treatment have reduced breast cancer mortality in the US over the past 30 years. However, the widespread adoption of screening mammography has been accompanied by dramatic increases in early stage breast cancer diagnoses that have not been offset by declines in advanced stage disease. Accumulating evidence suggests that a substantial fraction of screen-detected breast cancers would never have emerged clinically if not detected through screening. While there is extensive debate regarding the magnitude of overdiagnosis, there is widespread consensus that new approaches are urgently needed to distinguish indolent screen-detected cases from those that may be life threatening. The role of the tumor microenvironment in breast cancer progression has been increasingly recognized. Several lines of evidence indicate that breast tumorigenesis is critically influenced by active signaling between malignant breast epithelial cells and non-neoplastic cells of the tumor microenvironment. The goal of our proposal is to identify tumor microenvironment signatures that predict the aggressiveness of early stage, screen-detected breast cancers by minimally invasive methods. We will leverage and refine state-of-the-art technologies to characterize aggressive signatures based on the cellular composition and gene expression of specific cell populations within the tumor microenvironment of interval- and symptom-detected invasive breast cancers. We will then determine whether the presence of these aggressive tumor microenvironment signatures in early stage, screen-detected breast cancer is associated with progression. We will obtain retrospective data on 800 formalin-fixed, paraffin-embedded specimens for analysis from the Vermont Breast Cancer Surveillance System (VBCSS), which has collected integrated patient, radiology, pathology, treatment, and outcomes data on all women undergoing breast imaging in the state of Vermont since 1996. The VBCSS has a large existing repository of over 1,200 centrally-reviewed DCIS specimens and access to over 10,000 invasive breast cancer specimens for cases diagnosed in the state of Vermont. We will also engage in prospective collection of fresh specimens via the Vermont Cancer Center Tissue Biobank, which is also linked to the integrated data of the VBCSS. The identification of aggressive and indolent tumor microenvironment signatures will promote the development of more conservative treatment strategies for the subset of women with favorable prognosis and suggest novel targets for therapeutic intervention in cases with unfavorable prognosis. We have assembled a multidisciplinary research team with nationally recognized expertise in cellular and molecular cancer biology, pathology, cancer screening, and epidemiology, as well as a long track record of productive consortium-based collaborative research. The Vermont Breast Cancer Molecular Characterization Laboratory will provide the consortium with scientific leadership, technical resources, and access to a large repository of retrospective and prospectively collected breast specimens linked to the rich data of the VBCSS.